1. Field of the Invention
The present invention relates to a rotor oscillation preventing structure for a steam turbine.
2. Description of the Related Art
Steam turbines generally have a plurality of stages composed of moving blades and stator vanes in the axial direction of a turbine rotor as shown in FIG. 7. A gap exists between a moving blade and an outer circumferential side stationary wall and a portion of operating steam leaks through this gap. Leakage flow does not allow the moving flow to generate power; therefore, it results in a loss. In order to minimize the leakage, labyrinth seal fins are installed vertically to the axis of the turbine at the outer circumferential side stationary wall facing a shroud cover on the outer circumference of the moving blades. The labyrinth seal fins, along with the outer circumferential side stationary wall, form labyrinth seals.
Incidentally, it is known that if the turbine rotor becomes eccentric with respect to the outer circumferential side stationary wall, fluid force acts in an eccentric-vertical direction, which causes self-induced oscillation called steam whirl.
In the shroud cover and labyrinth seals installed on the outer circumferential side stationary wall facing the shroud cover in the steam turbine, the turbine rotor may become eccentric with respect to the outer circumferential side stationary wall. In such a case, fluid force acts on the rotor in an eccentric-vertical, rotational direction, and then the rotor is displaced to the eccentric-vertical rotational direction. In the position after the displacement, the fluid force acts again in the eccentric-vertical, rotational direction to repeat the displacement. In this way, the rotor whirls. This self-induced oscillation is steam whirl.
Steam whirl has been studied through the ages. It is found that a whirl component of leakage flow passing through labyrinth seals contributes to instability (see H. Benckert: “Flow induced spring coefficients of labyrinth seals for application in rotor dynamics”: NASA CP-2133: 1980).
Therefore, if the occurrence of steam whirl was predicted, measures for reducing whirl flow were adopted. However, to predict the occurrence of the steam whirl, the fluid force exerted on the rotor due to an eccentricity of several hundred μm has to be captured accurately. This is very difficult even if the most recent fluid analysis technologies are made full use of. Thus, as regards the steam whirl, it is preferable that the causes of the instability be excluded to the extent possible in a permissible range of cost with a safety factor ensured.
One of the conventional rotor oscillation preventing structures is a structure in which a whirl preventing plate is installed on an outer circumferential side stationary wall surface upstream of labyrinth seals in order to reduce the whirl component of leakage flow (see JP-2008-184974-A and JP-56-69403-A).